Current projectile launching weapons known in the art, such as the M327 120MM Rifled, Towered Mortar of the Expeditionary Fire Support System (EFSS), require multiple alignment components and tedious procedural adjustments to properly adjust the weapon's sight mount in order to accurately aim at a test target. For example, the M327 120MM mortar uses a sight unit and a boresight. The boresight represents the centerline of the bore, while the sight unit represents the location of the sight mount. With the reticle of the boresight aligned with the test target, using the level vials of the sight unit for additional adjustment, the reticle of the sight unit is aligned with the target within tolerance.
The sight mount therefore serves as the sole datum, or known, recognized reference point for aiming a mortar. All operations and procedures of aiming and firing the mortar function on the assumption that the sight mount is parallel to the centerline of the bore of the mortar. There is no way to check the sight mount parallelism while in the field, so it is imperative that a user can trust the sight mount calibration, usually completed by a maintainer during routine mortar maintenance. If the sight mount is out of tolerance, or not parallel, the mortar will not be aimed properly, which could result in injury or death of friendly personnel or damage and destruction of property.
Current boresights and sight units known in the art are limited in their capability to properly align a weapon with a target due to inherent errors occurring in the sight units, most of which stem from the fact that the sight unit is adjustable for field use and operations, and, while acceptable in the field, the level vials do not provide enough sensitivity to maintain tolerance objectives at the maintenance and calibration level.
The sight units use worm and bevel gears attached to measurement scales for mounting and operation, and these gears rely on spring tension to maintain proper gear engagement. While this is a common design known in the art for sight units to help mitigate the high impulse loads resulting from firing the weapon, the movement possible with existing sight units, combined with the backlash inherent in any gear train design, creates an inexact and unreliable basis for adjustment and calibration of the sight mount. The spring tension and backlash can also result in a potential loss of parallelism between the dovetail mounting surface and the telescope, since the telescope assembly to the sight unit contains multiple gear interfaces.
The level vials used in current sight units known in the art, such as the M67A1 sight unit, allow for up to ±5 mils error in adjustment, which is acceptable for mortar fire missions, but not for the precise and accurate measurements needed for sight mount adjustment. A “mil” or “gunner's mil” is a unit of measure of an angle and is the standard unit of measure for angles in the artillery field. There are 6400 mils in a 360° circle, making 1 mil equal to 0.00278°.
Because the level vials are also adjustable, further inherent error is introduced during adjustments.
The current sight unit also uses a screw traveling eccentric to the centerline of a locking collar for calibration of the level vials. While the eccentric adjustment approach allows for a more compact design and can hold an acceptable field-level tolerance, it makes level vial calibration exceedingly difficult, and opens the door for a host of calibration-related issues. In essence, an incorrectly calibrated sight unit results in an incorrectly calibrate sight mount.
Sight units known in the art are also cumbersome to use. Current sight units require two operators: one to turn the screws to adjust the sight mount, and one to read and give direction based on the bubble in the level vial. Communications between operators may also introduce additional difficulties to the alignment process.